1. Field of the Invention
This invention relates to tubular jet engines and pulse-jet engines of the valveless type, and more particularly to those that are capable of being throttled, easily self-starting without the use of auxiliary air, and self-aspirating. These engines typically consist of a combustor tube and an intake duct delivering the fuel-air charge. The combustor tube generally consists of a combustion chamber closed at its forward end, connected through a reducing cone, to an exhaust tail tube of lesser diameter than the combustion chamber. The exhaust tail tube is open to the atmosphere at its aft end. These engines normally operate on liquified gaseous fuels, such as propane, or butane, or vaporized liquid hydrocarbon fuels such as gasoline or kerosene. Valveless engines that use a fuel mist or spray, such as an atomized gasoline liquid, have very limited throttle capability and are not self-starting.
2. Description of the Prior Art
Most valveless jet engines of the prior art have the intake duct extending from the combustion chamber or reducing cone of the engine at a 45-90 degree angle to the longitudinal axes formed by the combustion chamber, reducing cone, and the exhaust tail tube. The intake duct and its inlet mouth typically face to the rear of the engine. At these angles, the intake duct represents a protrusion from the longitudinal plane, making it difficult to mount these engines to airplane structures, especially the fuselage of an airplane.
The history of the valveless jet engines covers several techniques to block the backflow of combustion gases out the intake during the combustion cycle. These pressure reversals or shock waves during combustion are difficult to eleminate entirely. They cause instability in the intake, preventing fuel flow to the combustion chamber, and often ignite the fuel-air mixture in the intake duct before it gets to the combustion chamber, resulting in total operating failure. They are also a cause of reduced thrust, because they reduce pressure build-up in the combustion chamber. Common methods described in the prior art to reduce backflow include: extending the length of the intake duct; sonically tuning the intake frequency to the frequency of the exhaust tail tube; and employing novel concepts such as vortex valves, flow rectifiers, or inertia tubes. None of these methods so far has resulted in complete blocking of the backflow, sufficient to be able to mount the intake duct on the front face of the combustion chamber with the inlet mouth of the intake duct facing forward in the direction of flight. One such engine with an intake duct mounted on the front face of the combustion chamber, the SNECMA valveless pulse-jet, known as the "Escopette," has the mouth of the duct turned back 180 degrees, so it faces the same direction as the exhaust tail tube, conserving the backflow for thrust. Another SNECMA engine, the "Ecrevisse," turns back the reducing cone 180 degrees so the intake duct and the exhaust tail tube face in the same direction. To achieve the same objective, another inventor turns back the exhaust tail tube 180 degrees. In U.S. Pat. No. 3,823,554, the inventor aligns the intake duct on a common longitudinal axis with the combustor tube and has the intake duct and inlet mouths facing directly aft in the same direction as the exhaust tail tube.
Mounting the intake duct on the front face of the combustor tube and on a common longitudinal axis with the combustor tube, and with the intake mouth facing the direction of flight, has several important benefits, among them, allowing ram air to enter the intake during flight, with a resultant increase in thrust and a reduction of specific fuel consumption. Almost all inventors of prior valveless jet engines have not been able to mount their intake ducts and intake inlet mouths facing forward, due to backflow of combustion gases out the intake duct. Instead, they have accepted a certain degree of spillage back through the intake duct, and have either turned back the intake duct 180 degrees to align the spillage flow with the exhaust tail, as mentioned above, or at least partly turned the intake duct back at an angle of 35-45 degrees to the axis formed by the combustion chamber and exhaust tube tail. Turning back the intake duct has the benefit of conserving the backflow of combustion gases through the intake duct for thrust, but it also eliminates the opportunity to maximize the positive effects of ram air to dramatically increase thrust and enrich the fuel charge with air. To obtain some benefits of ram air, inventors have employed scoops to redirect ram air into aft facing intake inlet mouths, or utilized forward facing scoops to direct ram air into an intake duct that is angled partly to the aft of the engine. Even with such scoops, it is not possible for engines of the prior art to realize the full benefits of ram air during flight. A forward facing intake duct aligned on a common axis with the combustor tube, and with the inlet mouth also facing forward in the direction of flight, achieves the highest air-fuel ratio during flight, and the greatest acceleration of the fuel-air charge to the combustion chamber. The advantage of having the fuel-air charge and combustion products all moving in one direction on a common longitudinal axis results in a dramatic improvement in thrust.
Another engine, the Gluhareff engine, specified in U.S. Pat.No. 3,093,962, handles backflow by allowing reverse pulses or shock waves caused by combustion gases to exit out the intake duct diffuser at a 90 degree angle to the axis of the combustor tube. At this angle, the exhaust gases exiting from the intake do not add to to the forward thrust of the engine, but reduce thrust by reducing pressure build-up in the engine. Although the Gluhareff invention has air scoops to realize some benefits from ram air, it is designed for mounting to, and driving helicopter blades; its bulky, long intake, mounted at 90 degrees to the combustor tube, make this engine very difficult to mount to the fuselage of an airplane.
It is worthy to note that there exists in the early prior art, several examples of valveless jet engines having the intake duct installed on the forward face of the combustor tube, and with the inlet mouth of the intake duct facing forward in the direction of flight. Namely, the Dunbar-Schubert engine, specified in U.S. Pat. No. 2,709,891, and the Bodine engine, specified in U.S. Pat. No. 2,731,795 are examples of such engines. These engines do not relate to the present invention because they are not self-starting, and their ability to be throttled is unknown. To start these engines, forward movement or auxiliary air is required. The Dunbar-Schubert engines also require a continuous supply of outside air to operate, which means they are not self-aspirating. Although both Dunbar-Schubert and Bodine claim their engines shunt off all blowback of combustion gases through their respective intakes, this inventor believes these engines do experience significant blowback of combustion gases. Both engines rely on adjusting the length of the intake duct in relation to the length of exhaust tail tube, otherwise known as sonic tuning, to eliminate backflow of combustion gases out the intake duct. However, extensive testing by this inventor as shown that this method leaves as much as 15% of the blowback unchecked, to exhaust out the intake duct. Even when the intake duct penetrates the combustion chamber, as discussed below, adjusting the intake duct length in relation to the length of the exhaust tail tube, or sonic tuning, does not eliminate all backflow of combustion gases out the intake duct. Due to their limitations, the Dunbar-Schubert and Bodine engines do not represent practical power sources for aircraft propulsion.